This invention relates to an optical interface arrangement in which an optical signal is received or transmitted by a photoelectric device. The optical signal will usually be brought to, or transmitted from, the optical arrangement by means of an optical fiber.
In the case of a receiver, the photoelectric device will usually be a photodiode, for which an electrical amplifier can be placed after the photodiode to boost the electrical signal if necessary.
Such receivers can be used in a wide range of analogue and digital systems to convert the optical signals present in the optical fiber into electrical signals. Digital applications include telecommunications systems. Analogue applications include remote control of microwave transmitters and optically-fed phased array antennas. In such applications the broad-band high-frequency signals are transmitted over optical fiber. The optical modulation frequencies and hence the electrical carrier frequencies of interest are in the microwave and millimeter wave region.
At such high frequencies the minimization of unwanted parasitic elements of the electrical components and electrical connections becomes very important.
In a conventional photodiode the optical signal is directed at its surface. Usually such a photodiode is placed on a ceramic tile which is made of dielectric material. Metal tracks on the surface of the tile carry the detected electrical signal. Electrical connection from the tile to the photodiode is made using solder contacts.
In a conventional optical receiver design, the photodiode tile is placed horizontally on a flat surface adjacent to the electrical amplifier with both being in the same plane. By making the components small and placing them close together the length of the connecting bondwire can be minimized to maintain a low parasitic inductance and therefore minimize its detrimental effects.
Because of the surface entry design of a conventional photodiode the light must enter or leave from a direction orthogonal to the photodiode surface. The interfacing of the optical input from the optical fiber to the photodiode can be performed in several ways. For example, the fiber can be placed such that its axis is orthogonal to the photodiode surface so that the end of the optical fiber is placed close (butt coupled) to the photodiode detecting region and no intervening optical coupling or focusing components are required. Unfortunately, in many applications it is desirable to place the input fiber and an output electrical connector in the same plane as each other and in the same plane as a substrate carrying electrical circuits including the amplifier.
In such a case, this can be achieved by leaving the photodiode tile in the same plane as the electrical circuits, and by deflecting the light through 90xc2x0 down onto the photodiode. The methods used for this technique could include the use of a focusing lens and prism, or a 45xc2x0 polished fiber end. The manufacture of components and the optical alignment of these methods are more difficult than that of the simple butt coupling method.
One way to avoid the 90xc2x0 deflection of the light into the photodiode is to place the photodiode tile at right angles to the substrate which carries the electrical circuits, but it is difficult to use conventional wire-bonding equipment to bond around such a 90xc2x0 comer, and also the bondwire can be excessively long, introducing too much parasitic inductance and hence degrading the performance. One known arrangement in which the photodiode plane is at right angles to that of the electrical circuits is disclosed in UK patent application GB 2185151A, in which the photodiode tiles are formed from a larger substrate which is sawn partway through in a series of parallel lines where the edge of the photodiode tiles will be. Patterning of electrically conductive tracks is then performed which pass around the tile edge into the troughs made by the partial sawing. The substrate is then turned over and sawn up into individual tiles by sawing the rest of the way through the substrate from the opposite face. Thus, the resultant track goes around the edge of the tile and continues partway along the edge of the tile. The length of the bondwire linking the photodiode to the electrical substrate is minimized by such an arrangement, but it has disadvantages in that a relatively thick phototile is required, and the electrically conductive tracks extend only partway along the edge thickness of the tile.
The present invention seeks to provide an improved optical interface arrangement. According to a first aspect of this invention, an optical interface arrangement comprises: a tile carrying an optical component which is mounted substantially at right angles to a closely adjacent substrate which carries an electrical circuit and which has an electrically conductive track extending to an edge of the substrate adjacent to an edge of said tile; the edge surface of said tile having a localized recess carrying an electrically conductive material which extends across the thickness of the tile and along a major surface of said tile to connect with said optical component, and a short bondwire linking the electrically conductive material in said recess to the adjacent electrically conductive track on the substrate.
According to a second aspect of this invention, a method of making an optical interface arrangement includes the steps of: forming an array of holes through a sheet of tile material; coating the walls of the holes with an electrically conductive material, dividing said sheet into a plurality of smaller tiles such that an edge of each tile passes through a hole to leave a recess on that edge which carries electrically conductive material; mounting an optical component on each tile so that it is electrically connected to said electrically conductive material; mounting each tile at right angles to an adjacent substrate which carries an electrically conductive track; and linking said electrically conductive track to said material in the recess by means of a short bondwire.